The present invention is related to fine denier polyacrylonitrile fibers, and in particular, fine denier polyacrylonitrile fibers obtained by splitting multicomponent fibers and to fabrics made from such fine fibers.
Filtration processes are used to separate compounds of one phase from a fluid stream of another phase by passing the fluid stream through filtration media, or septum, which traps the entrained or suspended matter. The fluid stream may be either a liquid stream containing a solid particulate or a gas stream containing a liquid or solid aerosol.
In recent years, particular emphasis has been placed on air filtration, in specific the filtration of respirable dust from air streams. It is now widely recognized that inhaled particulates, particularly particles in the sub-10 micron range, have adverse health effects. In 1970, the U.S. Environmental Protection Agency (EPA) set forth a National Ambient Quality standard for particulate matter directed at the reduction of respirable particles contained in emissions. Filters are widely used to control the particulate matter released in emissions because filters are reliable, efficient, and economical. For example, high efficiency particulate air (HEPA) filters and ultra efficiency particulate air (ULPA) filters have been developed which specifically target the removal of fine respirable particulates.
Fabrics are widely used as filtration media. Conventional filters remove particulates by physically obstructing the flow of particles of a given size or larger; i.e., by mechanical action. A fundamental dilemma in the use of fabrics in small particulate filtration is that conventional textile fibers, having fiber diameters of 20 microns or more, are relatively coarse in comparison to the particulates to be removed. These relatively thick fibers produce filter media having large interfiber pores. Such open, porous structures do not provide suitable interstitial configurations for efficiently trapping fine contaminant particles.
Extremely fine fibers are known to be beneficial in the filtration of extremely small particulates. These fine denier fibers may be used to produce fabrics having smaller pore sizes, thus allowing smaller particulates to be filtered from a fluid stream. In addition, fine denier fibers can provide a greater surface area per unit weight of fiber, also considered beneficial in filtration applications.
Meltblown technology is one avenue by which to produce such fine denier filaments. Fine denier meltblown webs have been widely employed as filter media because the densely packed fibers of these webs are conducive for providing high filter efficiency. However, meltblown webs typically do not have good physical strength, primarily because less orientation is imparted to the polymer during processing and lower molecular weight resins are employed. Thus, in general, meltblown filter media are laminated to at least one separate, self-supporting layer, which adds cost and complexity to the manufacturing process. Although the physical integrity of the meltblown web can be improved by increasing the thickness of the web, this in turn increases the pressure drop required to force air through the filter media. In addition to producing fine denier filaments, meltblown technology also typically yields shorter fibers than other fiber formation techniques. This is problematic because these short fibers cannot easily be entangled using conventional nonwoven web formation processes, such as hydroentangling and needlepunching.
Conventional melt extrusion processes can provide higher strength fibers than meltblown fibers. However, it is difficult to produce fine denier fibers, in particular fibers of 2 denier or less, using conventional melt extrusion processes. Therefore, while filter media produced from nonwoven webs of conventional textile fibers, such as spunbond and staple fiber webs, have been used in filtration applications such as stove hood filters, there is room for improvement in their use as filter media for fine particulates.
One avenue by which to produce fine denier fibers using conventional melt extrusion is to split multicomponent continuous filament or staple fiber into fine denier filaments, or microfilaments, in which each fine denier filament has only one polymer component. Multicomponent fibers, also referred to as composite fibers, may be split into fine fibers comprised of the respective components, if the composite fiber is formed from polymers which are incompatible in some respect. The single composite filament thus becomes a bundle of individual component microfilaments following splitting. See, for example, U.S. Pat. Nos. 5,783,503 and 5,759,926, reporting splittable multicomponent fibers containing polypropylene, such as splittable polyester/polypropylene and nylon/polypropylene fibers.
A number of processes are known for separating multicomponent fibers into fine denier filaments. The particular process employed depends upon the specific combination of components comprising the fiber, as well as their configuration. One common process by which to divide a multicomponent fiber involves mechanically working the fiber, by means such as drawing on godet rolls, needle punching or hydroentangling. The production of mechanically splittable multicomponent fibers presents challenges not encountered in the production of other types of composite fibers. In particular, when mechanical action is used to separate multicomponent fibers, the fiber components must be selected carefully to provide an adequate balance between adhesive and dissociative properties. In particular, poor bonding is known to facilitate the separation process. Conversely, the components should remain bonded during at least a portion of the downstream processing incurred in fabric formation. To add to this difficulty, many conventional textile processes, such as carding, impart significant stress to the fiber, thus promoting premature splitting. Premature splitting is highly undesirable because conventional textile equipment is frequently not designed to process extremely fine filaments, and quickly becomes fouled by them. In addition to their adhesive properties, the melt rheologies of the polymers comprising the multicomponent fiber also strongly influence the splitting process. For example, the melt rheologies of the two components must be such that one component does not totally encapsulate the other during melt spinning, thus precluding later splitting.
As an alternative to the use of fine denier fibers, the efficiency of filters may also be increased by utilizing electrets, generally defined as electrically non-conductive materials capable of storing an applied charge for a relatively long period of time. In particular, electret filters are known to have a higher filtration efficiency than a comparable neutral filter, with no greater resistance to air flow. This increase in efficiency is due to the fact that substantially all industrial processes produce both positively and negatively charged particulate matter. For example, energy intensive operations, such as grinding, are known to produce particles with extremely high levels of charge. It is generally accepted that these charged particles are electrostatically attracted to oppositely charged surfaces within an electret filter. Further, in contrast to traditional mechanical filtration, which occurs primarily at the surface of the filtration media, electret filters contain charged fiber surfaces throughout the filter thickness, thus providing a greater total surface area for filtration.
Many conventional polymers, such as those used in textile fibers, develop and retain charges on their surface for an extended period of time, thus forming electrets. A wide variety of polymers may be used to produce electret fibers, including polypropylene, polyethylene, nylon, acrylic, modacrylic and polytetraflouroethylene. In particular, nonwoven fabrics formed from polypropylene fiber are known for use in electret filters. Such filters are disclosed in U.S. Pat. Nos. 5,597,645 and 5,792,242. See also U.S. Pat. No. 4,874,399, reporting the additional benefits of a polypropylene polymer blend in electret filter applications. Polypropylene is attractive for use in filtration because, in addition to its electret properties, it is economical, insensitive to moisture, has adequate tensile properties, and superior chemical resistance. However, although electret filters comprised entirely of polypropylene are known, the charge developed in such filters is limited because each individual fiber carries both a positive and a negative charge. Single fiber electrets carry this dual charge on opposite sides of the fiber diameter. This is problematic because the strength of the overall charge which develops on a filament is dependent on fiber diameter, namely, smaller diameter fibers have lower maximum charge strength than larger diameter fibers. Several factors are involved in this phenomenon. Namely, opposite charges on a contiguous surface have a natural tendency to migrate towards each other over time, ultimately resulting in charge neutralization. Further, the driving force required for charge neutralization is inversely proportional to the distance across which the charges must bleed during neutralization and directly proportional to the strength of the opposite charges. Therefore, it is generally difficult to develop and/or retain significant amounts of charge in fine diameter single fiber electrets.
Mixed fiber electrets can be used to avoid charge neutralization and to allow higher charges to develop on the fiber surfaces. In particular, electret filters containing a blend of fibers that are separated on the triboelectric series are known to develop and retain a greater charge. The triboelectric series is a scale that ranks a material""s ability to donate or accept electrons. Such a series is provided for textile yarns by Smith and East in xe2x80x9cGeneration of Triboelectric Charge in Textile Fibre Mixtures, and Their Use as Air Filters,xe2x80x9d Journal of Electrostatics, 21 (1988), p. 81-98, hereby incorporated by reference. Depending on the triboelectric properties of two surfaces, electrons can migrate from the surface of one of the materials to the surface of the other during contact. When the two surfaces are subsequently separated, one surface loses electrons, becoming more positive, while the other surface gains electrons, becoming more negative. The amount of electrons transferred depends both on the triboelectric properties of the two materials (which also correlates with differences between their dielectric constants) and the amount of surface area which is in contact. The ability to charge electret materials via such contact or friction is referred to as triboelectification.
Mixed fiber electrets containing a blend of fibers at opposing ends of the triboelectric series is disclosed in various patents, including U.S. Pat. Nos. 5,888,274; 5,368,734; and 4,798,850. For example, U.S. Pat. No. 5,368,734 is directed to electret filters formed from a blend of polytetraflouroethylene (PTFE) fibers and nylon fibers. However, the use of PTFE fibers is cost prohibitive.
Smith and East disclose mixed fiber electret fabrics containing a blend of polypropylene and acrylic fibers. See P. A. Smith and G. C. East, xe2x80x9cGeneration Of Triboelectric Charge In Textile Fibre Mixtures, And Their Use As Air Filters, 21 Journal of Electrostatics,xe2x80x9d 81-98 (1988). The use of acrylic fibers, however, is problematic in the production of filtration media for filtering fine particulates. There are process limitations constraining the formation of multicomponent fibers that include an acrylic polymeric component. In this regard, acrylic fibers are typically produced using solution spinning, and it is not currently commercially possible to form a multicomponent fiber having both solution spun components and melt processable components. Thus, filters including both acrylic fibers and other fibers must be prepared by separately producing the respective monocomponent fibers and then blending the fibers when making the filler. However, while small diameter acrylic fibers can be made, as discussed above, such fibers typically do not have adequate tensile strength.
The present invention combines the benefits derived from both fine denier fibers and mixed fiber systems. The present invention provides splittable multicomponent fibers and fiber bundles that include a plurality of fine denier filaments having many varied applications in the textile and industrial sector. The multicomponent fibers and fine denier filaments can exhibit many advantageous properties, including the ability to develop and retain a significant level of charge on their surfaces for an extended period of time, and the like. The present invention further provides fabrics and filters, including electret filters, formed of the multicomponent fibers and fiber bundles, as well as processes by which to produce fine denier filaments and articles therefrom.
In particular, the invention provides splittable fibers having at least one component comprising a melt processable polyacrylonitrile polymer and at least one component comprising a polyolefin polymer, preferably polypropylene. The polymer components are dissociable by mechanical means to form a bundle of fine denier fibers. The multicomponent fibers prior to dissociation can have a variety of configurations, including pie/wedge fibers, segmented round fibers, segmented oval fibers, segmented rectangular fibers, segmented ribbon fibers, and segmented multilobal fibers. Further, the mechanically splittable multicomponent fibers can be in the form of continuous filaments, staple fibers, or meltblown fibers. The splittable fibers may be dissociated by a variety of mechanical actions, such as hydroentangling, carding, crimping, drawing, and the like.
The instant invention also provides a fiber bundle that includes a plurality of dissociated microfibers of different polymer compositions. Specifically the fiber bundle includes a plurality of polyacrylonitrile microfilaments and a plurality of polyolefin microfilaments, advantageously polypropylene microfilaments. In general, the microfilaments of the present invention range in size from 0.05 to 1.5 denier, and have a tenacity ranging from about 1.5 to about 4 grams/denier (gpd).
The multicomponent fibers of the present invention can be formed into a variety of textile structures, including nonwoven webs, either prior to or after fiber dissociation. Fabrics made using the fine denier fibers of the present invention are economical to produce and further provide superior characteristics, particularly when used as filter media. The fine denier fibers increase filter efficiency. In addition, a blend of fibers that differ significantly in their triboelectric characteristics, such as the blend of polyacrylonitrile fibers and polyolefin fibers, is beneficial in electret filters. Similarly, a blend of fine denier polyamide and polyolefin filaments is also beneficial in electret applications.
Fine denier fibers possessing superior tensile properties comprising polyacrylonitrile were not heretofore available. Surprisingly, the inventors have found that a multicomponent fiber which is both readily splittable, yet able to survive conventional textile processing intact, can be formed from polyacrylonitrile and a polyolefin, such as polypropylene.
Another aspect of the invention teaches fabrics formed from mechanically divisible multicomponent fibers comprised of at least one polyacrylonitrile polymer component and at least one polyolefin component, as well as the methods by which to produce such fabrics. In this aspect of the invention, the multicomponent fibers can be divided into microfilaments either prior to, during, or following fabric formation. Fabrics of the present invention may generally be formed by weaving, knitting, or nonwoven processes. Advantageously the fabric is a dry-laid nonwoven fabric formed from the multicomponent fibers of the present invention. Another advantageous fabric is a dry-laid nonwoven fabric bonded by hydroentangling.
Products comprising the fabric of the present invention provide further advantageous embodiments. Particularly preferred products include electret filtration media. In one aspect of this preferred embodiment of the invention, a nonwoven fabric comprised of the microfilaments of the present invention is needlepunched to charge the fabric structure, thus providing a superior electret filter medium.
Further understanding of the processes and systems of the invention will be understood with reference to the brief description of the drawings and detailed description which follows herein.